1. Field of the Invention
The present invention relates to apparatus for continuous reset-free variable polarization and phase control in an optical waveguide and has a plurality of function locations divided into pass sections of equal length.
2. Description of the Prior Art
The article by U. Heismann, R. Ulrich entitled "Integrated-Optical Single-Sideband Modulator and Phase Shifter", occurring in the IEEE transactions on Microwave and Technique, mTT-30 (1982), pages 613-617 discloses a known arrangement where the waveguide is composed of a strip waveguide indiffused in the surface of a lithium niobate crystal cut in the x-direction and the strip waveguide extends in the y-direction and perpendicular to the z-axis or optical crystal axis of the birefringent crystal. As a result, the TE mode and the TM mode have propagation constants .beta..sub.1 and .beta..sub.2, respectively, in the waveguide which are different from each other and which are, moreover, maximally different from each other due to such crystal material and such crystal cut.
With this arrangement, a pair of functional control locations is located in every length segment of the fixed period lengths whereby the spacing between the two functional element locations is smaller than the spacing between adjacent locations of functional pairs.
The fixed period length can be equal to the beat wavelength between the TE mode and the TM mode at the wavelength determined by the crystal material or can be an integer multiple thereof. In the disclosed embodiments, the period length is selected to be three times as large. Two coupling factors are alternately appointed to the successive functional locations with the one of these two coupling factors being proportional to sin .eta., the other being being proportional to cos .eta.=sin (.eta.+.pi./2).
In the embodiments according to the invention, the location of the functional elements are defined by comb-like interengaging electrode structures which are arranged above the waveguide on the substrate.
The known arrangement utilizes both the intrinsic birefringence as well as electro-optically induced birefringence having a small angle relative to the intrinsic birefringence refraction. The reset-free or continuous operation of the polarization conversion between the TE mode and TM mode with continuous polarization drift is achieved by application of the principle of the two-phase synchronous motor to integrated optics.
The wavelength .lambda..sub.0 of 0.6 .mu.m or 1.3 .mu.m determined by the crystal material causes extremely short beat wavelengths of 7 .mu.m or, respectively, 16 .mu.m and thus causes extremely short period lengths. For a complete polarization conversion between the modes polarized orthogonally relative to each other, the extremely short period length requires a large number of periods, i.e. line sections for this length. This results in an extremely narrow bandwidth arrangement in which the bandwidth is smaller tha 2 nm.
The article by C. H. von Helmolt, entitled "Broadband Single-Mode TE/TM Converters in LiNbO.sub.3 : A Novel Design", occurring in Electron. Letters 22 (1986) at page 155, describes a TE to Tm converter wherein a strip waveguide at the surface of a lithium niobate crystal cut in the y-direction extends at an angle of .theta.&lt;90.degree. relative to the x-axis of the crystal. Equidistantly spaced electrodes are arranged along the strip waveguide which are alternately supplied with positive and negative voltages. The sequence of the electrodes is divided into line sections of a fixed period length such that two electrodes are located in every line section.
As compared to a corresponding converter wherein the strip waveguide extends parallel to the x-axis and thus perpendicular to the crystal axis or z-axis of the crystal, this converter has a larger bandwidth. With an angle of .theta.=80.degree., for example, a relative bandwidth of (.lambda.-.lambda..sub.0)/.lambda..sub.0 of 3.2% is achieved at the prescribed wavelength of .lambda..sub.0 =1.3 .mu.m. Twenty-two line sections of the fixed period length are thus required for a complete polarization conversion. The fixed period length is about 0.55 mm and, thus, is 423 times the prescribed wavelength .lambda..sub.0.